1. Field of the Invention
The invention relates to a process for producing a valve metal powder by reducing a valve metal oxide, to valve metal powders obtainable in this way and to their use for producing solid electrolyte capacitors.
2. Brief Description of the Prior Art
Valve metals, especially those from transition groups 4–6 of the periodic system, and in particular tantalum and niobium, and alloys thereof, have numerous applications. One of the most important current applications for the above mentioned metal powders are solid electrolyte capacitors. Ta metal powders for this application are generally produced by Na reduction of potassium tantalum fluoride K2TaF7. Recently, reduction of the oxide has also been used to an increasing extent. In this context, reduction by means of gaseous reducing agents, to H2, alkali metal or alkaline earth metal, is preferred. In particular, magnesium vapor has proven to be a suitable reducing agent (WO 00/67936 A1, WO 00/15555 A1). These processes make it possible to produce high-quality valve metal powders, in particular tantalum and niobium powders, their alloys and their suboxides. All the processes described above prefer to use oxides in powder form, although other starting morphologies of the valve metal oxides or mixtures thereof which are to be reduced are also described. The desired physical properties and morphologies of the valve metal powders obtained by the reduction are adjusted by varying the reduction conditions or preferably by further treatment of the primary powders which originate from the reduction (e.g. WO 00/67936 A1, p. 9, lines 9 to 11).
WO 00/67936 A1 also describes a two-stage process for the reduction of niobium and tantalum pentoxide. In the first stage, the pentoxide is reduced using hydrogen, and in this way a corresponding suboxide is produced. The suboxide is then preferably sintered for 60 to 360 minutes at a temperature of greater than 1000° C., leading to stabilization of the crystal structure, i.e. of the primary structure of the suboxide. In the second stage, the suboxide is reduced to the metal using magnesium vapor. The morphology of the oxide, i.e. including of the secondary and tertiary structure, is not stabilized. This can be seen, for example, from Example 11, in which niobium pentoxide with a mean grain size of 1.7 μm is subjected to the two-stage reduction process described. The metal powder produced has a D50 value, determined using MasterSizer, of 160.9 μm, i.e. the mean grain size and therefore also the morphology have drastically changed.
The targeted adjustment of oxide morphologies is sufficiently well known (Heiko Thomas, Matthias Epple, Michael Froba, Joe Wong, Armin Reller, J. Mater. Chem., 1988, 8(6), pp. 1447–1451 and Lingna Wang, Mamoun Muhammed, J. Mater. Chem., 1999, 9, pp. 2871–2878). For example, DE 3918691 A1 has already described methods for setting defined primary grain sizes for oxides of niobium. It is also known to set defined agglomerate shapes and sizes. For example, processes for producing oxidic fibers and fabric produced therefrom, the production of defined agglomerates with particular properties, such as pore distribution (A. D. S. Costa, L. S. M. Traqueia, J. A. Labrincha, J. R. Frade, F. M. B. Marques, Third EURO-CERAMICS V.1, 1993, pp. 573–578), flow properties or pressure properties (T. Moritz, T. Reetz, Third EURO-CERAMICS V.1, 1993, pp. 633–638), as well as the production of platelets (Debojit Chakrabarty, Samiran Mahapatra, J. Mater. Chem. 1999, 9, pp. 2953–2957) or spherical particles (Hong Yang, Gregory Vovk, Neil Coombs, Igor Sokolov, Geoffrey A. Ozin, J. Mater. Chem., 1998, 8(3), pp. 743–750) have been described. Many metal oxide powders with corresponding oxide morphologies are even commercially available. The applications of metal oxides having defined oxide morphologies of this type are numerous, extending from spray powders for coating through pastes to applications in nanotechnology. There are also numerous processes used to produce such defined oxide morphologies. By way of example, mention may be made in the present context of the production of oxidic fibers via sol-gel chemistry and subsequent spinning of the gel.
A direct relationship between the morphology of the oxide used and a valve metal powder or its alloys or suboxides resulting from the reduction has not hitherto been described.